RESEARCH PERENNIAL GRASSES TRAITS AS FUNCTIONAL MARKERS OF GRAZING INTENSITY IN BASALTIC GRASSLANDS OF URUGUAY Martín Jaurena1*, Felipe Lezama2, and Pablo Cruz3 Natural grasslands in the basaltic region of Uruguay are threatened by an increase in stocking rates and changes in land use. To assess the effect of grazing intensification, plant functional types are proposed as simple tools to aid the monitoring and management of vegetation. In the present study we evaluated the effect of stocking rate increase at community level taking into account plant traits of 23 dominant perennial grass species. In order to identify plant functional types, we determined the grazing response in an experiment with two wethers stocking rates (0.78 and 1.56 livestock units ha-1) quantifying species cover and traits values. Leaf dry matter content (LDMC) and specific leaf area (SLA) were the traits that best described the perennial grasses response to the stocking rate increase and therefore are suggested to be used as functional markers. Three functional types were identified. Low stocking rates were related to functional type A (tall, warm season species with low SLA and high LDMC and functional type B (tall, cool-season species, with intermediate levels of leaf traits). On the other hand, high stocking rate encouraged functional type C (prostrate, warm season species, with high SLA and low LDMC). The classification of a highly diverse community into three functional types and the selection of traits as functional markers candidates is an innovative approach to develop simple and general methods to diagnosis the state of basaltic grasslands in Uruguay and to advise on its management. Key words: Natural grasslands, diagnosis, management, stocking rate. here is a growing interest to understand the impact of lowest in Uruguay (0.6 to 0.8 livestock units per hectare) different management practices on the sustainability and there are proposals to increase wool production using ofT grassland ecosystems in Uruguay. Although natural higher stocking rates. This vegetation is mainly composed grasslands represent the largest biome in the country (70% of perennial grasses of warm-season growing cycle with total area) and provide valuable economic and ecosystem both caespitose and prostrate habit, and to a lesser extent services, they are critically threatened by changes in land by perennial grasses of cool-season growing cycle that use and overgrazing (Díaz et al., 2006). Management of are mostly caespitose habit. However, the effects of grasslands with high floristic diversity and a wide range grazing intensification have not yet been evaluated. Many of productive systems should assemble environmental studies have described changes in botanical composition conservation and production concerns (Bullock et al., of these grasslands related to different stocking rates, 2001). In this context, increasing the abundance of but their results are difficult to use because they cannot perennial grasses (Dorrough et al., 2004) and particularly be extrapolated to other types of pasture or to different highly palatable cool-season grasses threatened by climatic or grazing conditions. Understanding the effects overgrazing (McIntyre and Lavorel, 2001) are important of grazing intensity on the dynamics of native grassland strategies for ensuring sustainability. communities is important to link animal production Natural grasslands are the typical vegetation of the with the conservation of the ecosystem services. Plant basaltic region of Uruguay that comprises one-quarter of functional types can be used as simple tools to aid the the country’s area. The carrying capacity of this area is the management and monitoring of the vegetation. Based on this context, we suggest that leaf traits and plant height 1Instituto Nacional de Investigación Agropecuaria INIA, Estación (PH) could be used to explain highly diverse basaltic Experimental del Norte, Ruta 5 km 386, Tacuarembó, Uruguay. grassland response to stocking rate increase and to classify *Corresponding author ([email protected]). the species responses into functional types. 2Universidad de la República, Facultad de Agronomía, Avenida The effect of grazing on species composition and Eugenio Garzón 780, Montevideo, Uruguay. diversity of grasslands remains controversial, but there 3Institut National de la Recherche Agronomique INRA UMR 1428 AGIR, BP 52627, 31326 Castanet-Tolosan, France. is a growing agreement that plant functional types can Received : 1 March 2012. predict the processes of grassland ecosystems (Díaz et Accepted : 15 September 2012. al., 2001; McIntyre and Lavorel, 2001). In this approach, 540 CHILEAN JOURNAL OF AGRICULTURAL RESEARCH 72(4) OCTOBER-DECEMBER 2012 CHILEAN JOURNAL OF AGRICULTURAL RESEARCH 72(4) OCTOBER-DECEMBER 2012 541 functional classification of grassland species tries to MATERIALS AND METHODS find groups of species with combinations of traits that show common responses to grazing intensity (McIntyre Site description and Lavorel, 2001; Sosinski and Pillar, 2004; Evju et Research was conducted at Glencoe Experimental Unit al., 2009). A similar response to treatments of grazing, (32o01´57” S, 57o13´52” W) of Instituto Nacional de mulching and burning was shown in different grasslands Investigaciones Agropecuarias (INIA) located in the although they were floristically different. In that case basaltic region of Uruguay. This region is part of the hemicryptophytes were the dominant life form in all of “campos” biome that includes East Central Argentina, the treatments that had in common the partial removal of South of Brazil, and Uruguay (Soriano, 1991). These biomass (Kahmen and Poschlod, 2008). The challenge of temperate grasslands are dominated by C4 perennial warm predicting community responses to grazing management season grasses and C3 cool-season grasses (Berretta, 2001; has promoted a search for key community parameters Jaurena et al., 2011). The mean annual rainfall is 1300 with emphasis on morphophysiological traits (Gitay and mm, evenly distributed throughout the year, with mean Noble, 1997; Díaz and Cabido, 1997; Lavorel et al., 1997; temperatures of 25 °C in summer and 12 °C in winter. The Westoby, 1998; McIntyre and Lavorel, 2001; Rodríguez present study was focused in Black Eutric Litosol soils et al., 2003). Traits such as PH, leaf area, leaf thickness with pH 6.1, organic C 2.9%, and a loam texture, which (Quadros and Pillar, 2001; McIntyre and Lavorel, 2001), averaged a DM production of 3800 kg ha-1 yr-1 over a 14- leaf tensile strength (LTS) (Boggiano, 1995) and leaf form yr period (Berretta et al., 2001). (Altesor et al., 1999) have been linked consistently with grazing intensity. A combination of height and other related Species grazing response traits can give a better indication of the different responses A portion of a database corresponding to surveys on black to mowing and grazing (Klimesova et al., 2008). Cruz et soils of a grazing experiment carried out by Jaurena et al. (2010) working with native grasslands of southern al. (2011) was used to determine the species grazing Brazil, showed that after 15 years of differential continuos response. This experiment studied from September 2006 grazing intensities it was possible to describe a gradient to November 2007 the effect of two contrasting stocking of grazing pressure by means of SLA and LDMC. These rates: 5.4 and 10.8 Merino wethers ha-1 (0.78 and 1.56 leaf traits are functional markers of species strategies for livestock units ha-1) in red and black shallow basaltic resource use (Ryser and Urbas, 2000; Garnier et al., 2004; soils, using two replicate paddocks of 0.55 ha alternately Al Haj Khaled et al., 2005; Li et al., 2005) and are a useful grazed (Jaurena et al., 2011). The herbage mass was approach for linking vegetation changes with ecosystem similar between treatments at the beginning but with function (McIntyre, 2008). The use of LDMC has been major differences at the end of the experimental period promoted as an indicator of resource use (Wilson et al., with 2150 and 1010 kg ha-1 pregrazing (Jaurena et al., 1999; Li et al., 2005) because it is related with growth 2011), and with 1604 and 537 kg ha-1 postgrazing for low rate (Gross et al., 2007). Besides that, LDMC is easy to and high stocking rate respectively. Of 83 taxa recorded measure, less variable between replicates and show lower in black soils, the 23 most abundant perennial grasses correlation with leaf thickness than SLA (Wilson et al., (seventeen warm-seasons and six cool-seasons) found 1999). with a minimum average cover of 0.4% were selected The objective of this work was to assess the response to assess its traits. The selection of species was based of the plant community associated with the stocking in the “biomass ratio hypothesis” proposed by Grime rate increase in basaltic grasslands of Uruguay, testing (1998). It postulates that ecosystem’s properties are the indicator value of plant traits and identifying plant related to species contribution to the total biomass of the functional types. To be more specific, we addressed the community (Garnier et al., 2004). This hypothesis implies following questions: I) Do plant traits of perennial grasses that the ecosystems functions are determined to a large explain the community changes related with the stocking extent by the trait values of the dominant species. Based rate increase in basaltic grasslands? We expect that the on this hypothesis, we evaluated only perennial grasses stocking rate increase may lead to changes in group of since they are the dominant group that explains 50 to 90% species frequencies related to the community response of forage biomass production at Rio de la Plata grasslands to grazing pressures. Therefore, common traits of this (Berretta, 2001; Cruz et al., 2010; Jaurena et al., 2011). species could be used as indicators of the community changes. II) Which functional types are related to grazing Species trait measurements intensities? We expect that the stocking rate increase In order to separate the effect of species replacement from changes the relative proportions of functional types.